It is proposed to study the vasopressin-containing system which terminates on the primary pituitary portal plexus. The cells of origin will be determined by stereotaxically placing radiofrequency lesions in selected hypothalamic sites and examining the brains by light microscopic immunohistochemistry for vasopressin and neurophysin. Whether or not the vasopressin-containing cells of the suprachiasmatic nucleus project to the portal capillaries, we will examine the ultrastructure of these cells and determine their sites of projection. To correlate changes in immunoreactive vasopressin and neurophysin in the zona externa of median eminence with rates of secretion, portal blood will be collected and assayed for these peptides. Comparisons will be made between levels in portal and peripheral blood. To determine if the axons on the portal capillaries represent a separate or collateral system to that terminating in the posterior pituitary, we shall examine changes in granule size and number as well as subcellular distribution of neurohormone at both neurovascular sites under a variety of experimental conditions, including adrenalectomy.